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This colorful view of Mercury was produced by using images from the color base map imaging campaign during Messenger’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury’s surface. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Considering the fact that Mercury wasn’t supposed to have explosive volcanism in the first place, the findings from a new analysis are surprising, and could have implications for understanding how the planet formed.

Mercury was long thought to be bone dry when it comes to volatiles, and without volatiles there can’t be explosive volcanism. But that view started to change in 2008, after NASA’s Messenger spacecraft made its first flybys of Mercury. Those glimpses of the surface revealed deposits of pyroclastic ash—the telltale signs of volcanic explosions—peppering the planet’s surface. What wasn’t clear from those initial flybys was the timeframe over which those explosions occurred.

For the study, researchers looked at 51 pyroclastic sites distributed across Mercury’s surface. They used data from Messenger’s cameras and spectrometers collected after the spacecraft entered orbit around Mercury in 2011. Compared with the data from the initial flybys, the orbital data provided a much more detailed view of the deposits and the source vents that spat them out.

The new Messenger data revealed that some of the vents have eroded to a much greater degree than others—an indicator that the explosions didn’t happen all at the same time.

“Together with other results that suggest the Moon may have had more volatiles than previously thought, this research is revolutionizing our thinking about the early history of the planets and satellites,” says Jim Head, professor of geological sciences and a Messenger mission co-investigator. “These results define specific targets for future exploration of Mercury by orbiting and landed spacecraft.”